The invention relates to high voltage current source circuits, and particularly to high voltage current source circuits that include bipolar transistors operating in their punch-through modes, and even more particularly to high voltage current source circuits that function as current sources from a positive supply voltge conductor.
It is well known that bipolar transistors ordinarily should not be operated in their various collector breakdown modes. The collector-to-emitter breakdown voltage BV.sub.CEO (base open circuited), BV.sub.CES (base electrode shorted to the emitter), and the collector-base breakdown voltage BV.sub.CBO (emitter open circuited) are well known and have been widely described in the literature. It is also known that bipolar transistors can be designed to operate in their "punch-through" modes. In the "punch-through" mode, the collector-base reverse bias voltage is increased to the point at which the portion of the depletion region of that junction within the base region extends all the way to the emitter-base junction. The punch-through voltage of bipolar transistors has been widely discussed and analyzed in the literature. Only if the magnitude of the punch-through voltage V.sub.PT is less than or equal to the lower of the breakdown voltage BV.sub.CES or BV.sub.CEO does "punch-through" of the transistor occur. Ordinarily, bipolar transistors are designed or selected so that collector-to-emitter breakdown and collector-base breakdown are avoided during normal circuit operation. However, the collector-to-emitter punch-through voltage has, on occasion, been used to advantage in some circuits. For example, U.S. Pat. No. 3,571,630 shows a regulator circuit in which the transistor operates in its punch-through mode to serve as a voltage reference.
High voltage circuits, such as high voltage, high power operational amplifiers, frequently need to include high voltage constant current source circuits that operate properly when subjected to very high supply voltages. For certain high power, high voltage bipolar operational amplifiers, lateral PNP current sources connected to the positive power supply have been widely used. Such constant current source circuits are commonly of the "current mirror" variety and are composed of matched lateral PNP transistors. The circuitry to which the constant currents are supplied by such lateral PNP transistor current mirror circuits may very widely, sometimes increasing collector voltage of a lateral PNP transistor enough to cause breakdown of that lateral PNP transistor. This condition is very undesirable, because it can change the bias voltage supplied not only to the lateral PNP transistor undergoing breakdown, but to all other current source transistors biased by the same voltage. This situation would probably result in total inoperability of the integrated circuit.
Since lateral PNP transistors and integrated circuits do not have nearly as high breakdown voltages as vertical NPN transistors normally fabricated on the same semiconductor chips, the breakdown voltages of lateral PNP transistors often are the limiting factors in determining the maximum magnitudes of the power supply voltages applied to integrated circuits.
Another shortcoming of lateral PNP constant current source current mirrors has been the fact that their collector or output impedances have not been as high as would be desirable, due to the inherently low gain of lateral PNP transistors. In order to increase the output impedance of the lateral PNP constant current source circuits, it is known to add a second lateral PNP transistor in "cascode" connection to the first one, connecting the emitter of the second lateral PNP transistor to the collector of the first one, and appropriately biasing the base of the second lateral PNP transistor. This greatly increases the output impedance of the collector of the second lateral PNP transistor, which then supplies the desired constant current. Such cascode circuits do not, however, significantly increase the breakdown voltage of the resulting circuit.
Although it would be possible to insert a zener diode between the collector of a conventional lateral PNP constant current source and the circuit to which the constant current is supplied, reducing the voltage applied across the collector-base junction and the voltage applied between the emitter and collector terminals of the lateral PNP transistors, this would preclude the possibility of operating the integrated circuit at low voltages and would limit the range of voltage levels the circuit requiring the constant current is allowed to have, for example, common-mode swing of an input amplifier. It is highly desirable that integrated circuits designed to have the capability of high voltage operation also be able to operate effectively when low power supply voltages are applied, and not have overly restricted internal voltage ranges.
There remains an unmet need for a means of increasing the breakdown voltage of a lateral PNP constant current source transistor in a conventional integrated circuit.